The present invention relates to a dual function probe for measuring combustion turbine exhaust characteristics. More specifically, the present invention relates to a dual function probe for measuring blade path temperature at the exhaust cylinder diffuser inlet of a combustion turbine and for extracting exhaust flow gases therefrom to facilitate emissions analysis.
In an axial flow combustion (gas) turbine, the hot gas exhausting from the last row of turbine blades is directed through an exhaust diffuser. The exhaust diffuser is formed by inner and outer flow liners disposed between an exhaust cylinder and a bearing housing. The flow liners serve to create a smooth flow path for the hot gas. They also act as a barrier which prevents the flow of hot gas directly over the exhaust cylinder and bearing housing, thereby preventing excessive temperatures and thermal stresses in these components.
Efficient operation of a combustion turbine requires constant monitoring of various aspects of the combustion process. One parameter of the combustion process that effects combustion efficiency is the blade path temperature, i.e., the temperature in the exhaust cylinder just aft of the last row of turbine blades. It is desirable, therefore, to periodically sample the blade path temperature in the combustion turbine.
Blade path temperature is usually measured using one or more thermocouple assemblies that are inserted through corresponding guides in the exhaust cylinder of the turbine such that the measuring junctions of the thermocouple assemblies are positioned within the flow path of the turbine exhaust near the last row of turbine blades. Typically, multiple thermocouple assemblies are inserted at equally spaced points around the annulus of the exhaust cylinder. Each thermocouple provides a blade path temperature reading for a respective one of the turbine's combustors. The measuring junction of each thermocouple is situated downstream from a respective combustor near the turbine blade aft annulus. In this manner, blade path temperature can be monitored for each combustor separately.
FIG. 6 provides a detailed view of a prior art thermocouple assembly 100 for use in measuring blade path temperature. As shown, the thermocouple assembly 100 comprises a thermocouple element 103 partially disposed within a first tubular member 101 (sometimes referred to as a "thermowell") so that the measuring junction 105 of the element 103 protrudes slightly from the forward end face 116 of the tubular member 101. The tubular member 101 has a narrow bore 104 at its forward end to provide a close fit with the thermocouple element 103. A slightly larger bore 102 extends through the remainder of the tubular member. The rearward end 115 of the first tubular member is coupled to an aspirating tee 112 which, in turn, is coupled to a second tubular member 111. A terminal head assembly 110 is coupled to the far end of the second tubular member 111.
As shown, the thermocouple element extends rearward of the assembly 100 through the aspirating tee 112 and second tubular member 111 to the terminal head 110. The leads of the thermocouple element are terminated on a terminal block 109 in the terminal head assembly 110. Extension wires (not shown) may be used to electrically connect the terminated thermocouple leads to a supervisory control device which can process the thermocouple data to calculate the temperature of the measuring junction 105. A male mating fixture 113 is integrally formed at the rearward end 115 of the first tubular member 101 for coupling the thermocouple assembly 100 to one of the guides in the exhaust cylinder after inserting the forward end 116 of the tubular member 101 through the guide and into the flow path of the turbine exhaust gases. Thus, in use, the aspirating tee 114, second tubular member 111 and terminal head assembly 110 remain external to the exhaust cylinder of the turbine.
Although the narrow bore 104 provides a close fit with the thermocouple element 103, when the forward end 116 of the assembly 100 is positioned in the flow path of the turbine exhaust, some of the exhausting gases may inadvertently enter the first tubular member 101 through the narrow bore 104. To prevent these extremely hot gases from escaping past the first tubular member 101 and damaging the terminal head assembly 110, an high temperature sealing gasket 106 is used to seal the rearward end 115 of the tubular member 101. The gasket 106 is forcibly seated against the end face of the large bore 102 by a stop ring 107 which is fabricated to the gasket 106 and spring biased toward the end face of the larger bore 102 by a spring 108 in the terminal head assembly 110. As shown, the thermocouple element 103 extends through the center of the gasket 107 without breaking the seal.
During operation, if the sealing gasket 106 is not properly seated against the end face of the bore 102, some exhaust gases may leak past the sealing gasket 106 into the second tubular member 111. The aspirating tee 112 operates to vent any leaked gas out of the second tubular member 111 to reduce the risk of damage to the terminal head assembly 110. Typically, a short piece of tubing (not shown) is connected to the outlet 114 of the aspirating tee 112 to direct the leaked gases to an innocuous location on the outside of the exhaust cylinder, such as, for example, under an insulating blanket surrounding the exhaust cylinder. Clearly, however, the narrow bore 104 and spring-loaded sealing gasket 106 of the prior art thermocouple assembly 100 are designed respectively to prevent any exhaust gases from entering into or escaping from the first tubular member 101. Should the sealing gasket 106 fail, the aspirating tee 112 merely serves as a protective bleed to prevent damage to the terminal head 110 and second tubular member 111.
With the increasing emphasis on the environment, it is important to ensure that the turbine exhaust does not exceed acceptable emissions levels. Measuring exhaust emissions is therefore an important aspect of combustion turbine operation. In the past, emissions levels have been monitored at the turbine exhaust stack using sensors or probes inserted into the exhaust stack. Because the exhaust stack is situated downstream from the individual turbine combustors, the individual emissions level of each combustor cannot be determined at the exhaust stack. It would therefore be desirable to provide a method or apparatus for extracting flow path gases from the exhaust cylinder of the turbine at a position much closer to the individual turbine combustors.
One possible solution would be to remove the thermocouple assemblies from their respective guides and replace the thermocouple assemblies with gas extraction probes. Exhaust gases flowing directly downstream from each combustor could then be extracted and independently analyzed. Because the thermocouple assemblies would have to be removed, however, blade path temperature readings could not be obtained during emissions testing.
Another possible solution would be to machine an additional set of guides through the exhaust cylinder so that gas extraction probes could be inserted into the flow path proximate the thermocouple assemblies. Although this would allow blade path temperature readings to be maintained during emissions testing, the additional set of probe guides would unacceptably increase the production costs of the turbine.
It would therefore be desirable to provide a means for extracting flow path gases from the exhaust cylinder of a combustion turbine near the turbine blade aft annulus without interrupting blade path temperature readings and without requiring modifications to the exhaust cylinder.
SUMMARY OF THE INVENTION
Accordingly, it is the general object of the current invention to provide a means for extracting flow path gases from the exhaust cylinder of a combustion turbine near the turbine blade aft annulus without interrupting blade path temperature readings and without requiring modifications to the exhaust section of the turbine.
Briefly, this object, as well as other objects of the present invention, is accomplished in a dual function probe that provides both a blade path thermocouple function and the ability to extract exhaust flow path gases from the combustion turbine. The dual function probe of the present invention is designed to insert into an existing thermocouple assembly guide in the exhaust cylinder of the turbine.
According to a preferred embodiment of the present invention, the dual function probe comprises an elongate tubular member having an open end and a sealed end. At least a portion of the tubular member is adapted for insertion through an existing thermocouple assembly guide and into the flow path of the turbine exhaust. An elongate thermocouple element is centrally disposed within the tubular member. The inner diameter of the tubular member is sufficiently larger than the diameter of the thermocouple element to allow exhaust gases to enter the open end of tubular member. An on-off valve is coupled to the tubular member proximate its sealed end. When the valve is open, exhaust flow gases that enter the open end of the tubular member are able to exit the tubular member through the valve. The terminal end of the thermocouple element extends outward through the sealed end of the tubular member to connect with an external control device.
In a most preferred embodiment, a union tee is used to couple the on-off valve to the elongate tubular member. An inlet of the union tee is coupled to the rearward end of the tubular member, and the terminal end of the thermocouple element extends outward through a first outlet of the union tee. The first outlet of the union tee has a reduced diameter substantially equal to the diameter of the terminal end of the thermocouple element thereby forming a seal around the terminal end of the thermocouple element. The sealed outlet defines the sealed end of the tubular member. The on-off valve is coupled to a second outlet of the union tee.
Preferably, the measuring junction of the thermocouple element protrudes slightly from the open end of the tubular member and comprises a duplex type measuring junction. A support plate may be coupled to the open end face of the tubular member to support the thermocouple element in its centrally disposed position within the tubular member. A sleeve may be slidably fitted over the outer surface of at least a portion of the tubular member to provide additional support and vibration damping to the forward end of the tubular member when the probe is inserted into one of the exhaust cylinder guides. A vacuum pump may be coupled through interconnection tubing to the outlet of the on-off valve to facilitate extraction of exhaust flow gases from the tubular member. A chemical analyzer may be coupled to the outlet of the on-off valve for analyzing the exhaust flow gases extracted through the valve. Damage to the chemical analyzer from the extreme heat of the extracted gases may be prevented with a cooling device coupled between the valve and the chemical analyzer.